Acoustic lens for micromachined ultrasound transducers

ABSTRACT

Matching layers configured for use with ultrasound transducers are disclosed herein. In one embodiment, a transducer stack can include a capacitive micromachined ultrasound transducer (CMUT), an acoustic lens, and a matching layer therebetween. The matching layer can be made from a compliant material (e.g. an elastomer and/or an liquid) and configured for use with CMUTs. The matching layer can include a bottom surface overlying a top surface of the transducer and a top surface underlying a bottom surface of the lens.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/205,123, filed on Mar. 11, 2014, now U.S. Pat. No. 9,502,023,entitled “ACOUSTIC LENS FOR MICROMACHINED ULTRASOUND TRANSDUCERS,” whichclaims benefit of U.S. Provisional Patent Application Ser. No.61/793,124, filed on Mar. 15, 2013, and entitled “ACOUSTIC LENS FORMICROMACHINED ULTRASOUND TRANSDUCERS,” both of which are herebyincorporated herein in their entireties by reference.

TECHNICAL FIELD

The disclosed technology relates generally to ultrasound transducers,and more specifically matching layers for ultrasound transducers.

BACKGROUND

In ultrasound imaging devices, images of a subject are created bytransmitting one or more acoustic pulses into the body from atransducer. Reflected echo signals that are created in response to thepulses are detected by the same or a different transducer. The echosignals cause the transducer elements to produce electronic signals thatare analyzed by the ultrasound system in order to create a map of somecharacteristic of the echo signals such as their amplitude, power, phaseor frequency shift etc. The map therefore can be displayed to a user asimages.

One class of transducer is a Micromachined Ultrasound Transducer (MUT),which can be fabricated from, for example, silicon and configured totransmit and receive ultrasound energy. MUTs may include CapacitiveMicromachined Ultrasound Transducer (CMUTs) and PiezoelectricMicromachined Ultrasound Transducer (PMUTs). MUTs can offer manyadvantages over other conventional transducers such as, for example,lower cost of production, decreased fabrication time, and/or widerfrequency bandwidth. MUTs, however, can be fragile and are typicallyutilized in single-use internal ultrasound imaging applications.

The use of a transducer in an external probe generally involves bondingor otherwise attaching an acoustic lens to the transducer. The acousticlens can protect the transducer from damage and/or may also provideacoustic focusing into a subject. In some low frequency applications,MUTs may be utilized in external probes having acoustic lenses madefrom, for example, an elastomer material. However, these elastomerlenses may not be suitable for high frequency ultrasound applications(e.g., greater than about 15 MHz) due to, among other reasons, increasedacoustic attenuation of the materials at the higher frequencies.Accordingly, a need for a low-loss and durable acoustic lens exists foran external MUT probe suitable for use at higher frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side schematic view of a prior art Capacitive MicromachinedUltrasound Transducer.

FIG. 2A is an isometric front view of an ultrasound transducer stackconfigured in accordance with one or more embodiments of the disclosedtechnology.

FIG. 2B is an enlarged side view of an ultrasound transducer stackconfigured in accordance with an embodiment of the disclosed technology.

FIGS. 3A and 3B are side views of an ultrasound transducer stackconfigured in accordance with an embodiment of the disclosed technology.

FIGS. 4A and 4B are side views of an ultrasound transducer stackconfigured in accordance with an embodiment of the disclosed technology.

DETAILED DESCRIPTION

The present technology is generally directed to matching layersconfigured for use with ultrasound transducers. It will be appreciatedthat several of the details set forth below are provided to describe thefollowing embodiments in a manner sufficient to enable a person skilledin the relevant art to make and use the disclosed embodiments. Severalof the details described below, however, may not be necessary topractice certain embodiments of the technology. Additionally, thetechnology can include other embodiments that are within the scope ofthe claims but are not described in detail with reference to FIGS. 1-4B.

Capturing ultrasound data from a subject using an exemplary transducerstack generally includes generating ultrasound, transmitting ultrasoundinto the subject, and receiving ultrasound reflected by the subject. Awide range of frequencies of ultrasound may be used to captureultrasound data, such as, for example, low frequency ultrasound (e.g.,less than 15 MHz) and/or high frequency ultrasound (e.g., greater thanor equal to 15 MHz) can be used. Those of ordinary skill in the art canreadily determine which frequency range to use based on factors such as,for example, but not limited to, depth of imaging and/or desiredresolution.

The disclosed transducers can be operatively connected to an ultrasoundimaging system for the generation, transmission, receipt, and processingof ultrasound data. For example, ultrasound can be transmitted, receivedand processed using an ultrasonic scanning device that can supply anultrasonic signal of any frequency. In some aspects of the presentdisclosure, an ultrasound system or device capable of operating at 15MHz or above can be used, while in other aspects ultrasound systems ordevices configured to operate below 15 MHz may also be used. While thetransducers disclosed below may be used in ultrasonic medicalmeasurement and/or imaging applications, they are not limited to suchuses. In some embodiments, for example, the transducers below can beused in biometric applications, such as, for example, fingerprintscanners.

FIG. 1 is a side view of an ultrasound transducer 100 configured inaccordance with an embodiment of the disclosed technology. Thetransducer 100 includes an electric power source 101 coupled to a topelectrode 104 and a bottom electrode 102 deposited on a substrate 114(e.g., a silicon substrate). The top electrode 104 is coupled to orotherwise adjacent to a membrane 108. As explained in further detailbelow, the top electrode 104 can be configured to cause deflections inthe membrane 108, which can, for example, cause an ultrasound wave topropagate therefrom. A gap 110 allows the membrane 108 to deflectsufficiently downward (e.g., toward the bottom electrode 102) withoutcoming into contact with the bottom electrode 102 and/or the substrate114. The membrane 108 may deflect in response to, for example, a changein voltage between the bottom and top electrodes 102 and 104 and/oracoustic energy (e.g. ultrasound waves) incident on the membrane 108.

In the illustrated embodiment, the transducer 100 is configured as aCapacitive Micromachined Ultrasound Transducer (CMUT). As those ofordinary skill in the art will appreciate, a bias voltage may be appliedby the power source 101 to the top electrode 104 and the bottomelectrode 102. The power source 101 can include an alternating currentsource and/or a direct current source (not shown). Acoustic energy(e.g., ultrasound waves) striking the transducer 100 can deflect themembrane 108, causing variations in the voltage between the top andbottom electrodes 102 and 104 to generate an electric signal.Conversely, applying an alternating current signal between the top andbottom electrodes 102 and 104 can deflect the membrane 108 to generatean ultrasound signal that can propagate away from the transducer 100.

FIG. 2A is a isometric front view of an ultrasound transducer stack 200configured in accordance with embodiments of the disclosed technology.The transducer stack 200 includes a transducer layer 201 below a firstlayer 220 and a second layer 224. The transducer 201 may comprise, forexample, a single array element, one-dimensional array of transducerelements, or a multi-dimensional array of transducer elements. Moreover,the transducer 201 may be made from any suitable transducer known in theart, such as, for example, piezoelectric transducers, CMUTs,piezoelectric micromachined ultrasound transducers (PMUTs), etc. Atransducer top surface 203 underlies a first layer bottom surface 223,and a first layer top surface 221 underlies a second layer bottomsurface 225. A second layer top surface 226 can be applied to or placedproximate to a subject (e.g., a human, an animal, etc.).

FIG. 2B is a side view of the transducer stack 200 configured inaccordance with an embodiment of the disclosure. In the illustratedembodiment, the transducer layer includes a CMUT (e.g., the transducer100 of FIG. 1). As those of ordinary skill in the art will appreciate,CMUT transducers (e.g., the transducer 201) can be fragile and may lackthe durability of other types of transducers (e.g., PZT transducers).However, placing a low acoustic loss and durable stiff material directlyonto a CMUT transducer can significantly reduce the efficiency of thearray and may prevent the array from even functioning. Accordingly, asdescribed in further detail below, assembling an ultrasound transducerstack with a relatively thin compliant layer (e.g., the first layer 220)between the lens (e.g. the second layer 224) and the transducer (e.g.,transducer 201) allows the transducer to emit ultrasound efficientlywhile also bonding the transducer 201 to the second layer 224.Furthermore, bonding a stiff outer layer can also maintain the flatnessof the transducer, thereby improving transducer efficiency and accuracy.

Referring to FIGS. 2A and 2B together, the first layer 220 is made froma compliant material (e.g., a PDMS-type silicone) configured to join orotherwise couple the second layer 224 to the transducer 201. In otherembodiments, however, the first layer 220 can be made from, for example,any material suitable for matching layers known in the art, such as, forexample, an elastomer, a gel, a polymerized material, etc. In furtherembodiments, the first layer 220 may be made from any suitable fluidsuch as, for example, water, an oil, etc. In some embodiments, forexample, the first layer 220 may be a thin, low-stiffness layer with alow Young's modulus, (e.g., less than 100 MPa).

As shown in FIGS. 2A and 2B, the second layer 224 can be configured asan acoustic window disposed on or near the first layer 220. As those ofordinary skill in the art will appreciate, the second layer 224 can be alow acoustic loss, durable layer and can robustly protect the transducer201 from impacts and/or exposure to contaminants while also protecting asubject (e.g., a human patient, a small animal, etc.) from a excessiveheat and/or charge produced by the transducer 201. The second layer 224may be made from any lens material known in the art suitable for usewith ultrasonic imaging such as, for example, a plastic, a plasticcomposite, a polymer, etc. In some embodiments, for example, the secondlayer 224 may be made from a thermoset cross-linked styrene copolymer(e.g., Rexolite) and/or polymethylpentene (e.g., TPX).

While the illustrated embodiment of FIGS. 2A and 2B is shown with onlythe first layer 220 and the second layer 224, more than two layers maybe alternatively utilized in accordance with the disclosed technology.In some embodiments, for example, at least a third layer (not shown)with an acoustic impedance approximately between a first acousticimpedance of the first layer and a second acoustic impedance of thesecond layer. In other embodiments, for example, a thin composite oflayers (not shown) can be used or incorporated into the transducer 200between the first layer 220 and second layer 224. The layers within thecomposite of layers can include acoustic impedances that graduallychange (e.g., increasing or decreasing) from the first acousticimpedance to the second acoustic impedance to improve the acousticimpedance matching between the first layer 220 and the second layer 224.

In some embodiments, the first layer 220 may have a thickness less than¼ wavelength of a ultrasound frequency range of interest. In otherembodiments, however, the first layer 220 can have a thickness anysuitable fraction (e.g., 1/1, ½, ¼, ⅛, etc.) of the wavelength ofultrasound frequency of interest. In some embodiments, for example, thethickness of the first layer 220 may be chosen to be suitably thin toreduce attenuation through the first layer 220 while having a suitablethickness to allow the movement of the membrane of the transducer 201and without being inhibited by the second layer 224. Moreover, in theillustrated embodiment, the second layer 224 is shown having a secondthickness T2 greater than a first thickness T1 of the first layer 220.In other embodiments, however, the first thickness T1 may have athickness equal to and/or greater than the second thickness T2. In somefurther embodiments, the first layer 220 can have varying thicknessbased upon, for example, performance characteristics (e.g., MUT membranethickness, cell structural characteristics, etc) and/or frequency ofultrasound to be emitted from the transducer 201. In some embodiments,for example, the second layer 224 can be configured to be removablyattached to the first layer 220 such that a plurality of differentlayers 224 (not shown) can be attached to the transducer 201 and thefirst layer 220.

As those of ordinary skill in the art will appreciate, directly bondingthe second layer 224 to the transducer 201 may prevent or reduce theemission of ultrasound energy from the transducer 201. For example,disposing the second layer 224 in direct or near contact with thetransducer 201 could significantly impede the movement of the membrane108 (FIG. 2A) in response to changes in the alternating current in thearray. Accordingly, placing the first layer 220 (e.g., a layer made froma compliant material) between the transducer 201 and the second layer224 can allow movement of the membrane 108 while also improving anacoustic impedance match therebetween. In some embodiments, the firstlayer 220 may also be configured to adhere, bond, or otherwise couplethe second layer 224 to the transducer 201.

In some other embodiments, for example, the transducer stack 200 caninclude additional layers. For example, an interstitial layer betweenthe first layer 220 and the transducer 201 can include a thin materialor a coated substance on the transducer 201 that can protect thetransducer 201 from corrosion while being sufficiently thin to notsignificantly affect the performance of the transducer 201. The firstlayer 220 can comprise, for example, water or any other suitable liquidhaving an acoustic impedance that is relatively similar to the acousticimpedance of the second layer 224.

As those of ordinary skill in the art will appreciate, an acousticimpedance mismatch between the first layer 220 and the second layer 224may cause reverberation and/or ring echoes. One way to reduce the impactof acoustic impedance mismatches is to create a textured interfacebetween the first layer 220 and the second layer 224, as shown in, forexample, FIG. 3A. Another way to reduce the impact of acoustic impedancemismatches is to configure and/or select the first layer 220 to have anacoustic impedance at least generally close to the acoustic impedance ofthe second layer 224 or vice versa. For example, one approach is to formthe first layer 220 from a composite material that has similar acousticimpedance as the material from which the second layer 224 is formed. Oneof ordinary skill in the art will know that adding particles of a moredense material to the chosen first layer material can increase thedensity of the resulting composite and therefore the acoustic impedanceas well. For example, in one embodiment, sub-micron particles can bedoped into the first layer 220 to increase or decrease the mass orotherwise vary the density of the first layer 220 to be as matched asclosely as possible to the second layer 224. In some other embodiments,for example, the first layer 220 can be doped with a plurality ofmicron-sized particles and a plurality of nano-sized particles. Forexample, the first layer 220 can include a low stiffness compliantmaterial, such as, for example, silicone, and a micron-sized powder canbe added to the silicone. However, merely adding the micron-sizedparticles to the first layer 220 may cause the micron-sized powder tosettle at the bottom of the first layer 220. Accordingly, a secondpowder with nano-sized particles can be added to the first layer 220 tofill in the spaces between the various micron-sized particles. This isdescribed in further detail below with reference to FIGS. 4A and 4B.

FIG. 3A is a side view of a transducer stack 300, configured inaccordance with an embodiment of the disclosure. A first layer 320couples a second layer 324 to the transducer 201. A textured surface 325of the second layer 324 overlies and/or contacts a top surface 321 ofthe first layer 320, and includes a plurality of grooves 322 and aplurality of ridges 323. The ridges 323 are configured to extend into aportion of the thickness of the first layer 320. In the illustratedembodiment, the grooves 322 and the ridges 323 extend longitudinallystraight across the second layer 324. In other embodiments, however, thegrooves 322 and the ridges 323 may have other patterns, such as, forexample, helical, diagonal, zig-zag, etc.

As those of ordinary skill in the art will appreciate, the texturedsurface 325 can reduce acoustic impedance mismatches in the transducerstack 300 by providing a graduated interface between the first layer 320and the second layer 324. The textured surface 325 may also, in someembodiments, improve adhesion between the first layer 320 and the secondlayer 324. As described above with reference to the first layer 220, thefirst layer 320 can be made from, for example, a compliant material,such as, for example, silicone or another suitable material with a lowstiffness that can be bonded to the transducer 201.

FIG. 3B is a side view of a transducer stack 301, configured inaccordance with an embodiment of the disclosure. A first layer 340couples a second layer 344 to the transducer 201. A bottom surface 345of the second layer 344 overlies and/or contacts a top surface 342 ofthe first layer 340. The bottom surface 345 includes a plurality ofpeaks 346 and a plurality of troughs 347 with a plurality of grooves 348formed therebetween. In the illustrated embodiment, the troughs 347extend from the second layer 344 into the first layer 340 isapproximately equal to the thickness of the first layer 340. In otherembodiments, however, the troughs 347 may only extend a portion of thethickness of the first layer 340.

FIG. 4A is a side view of a transducer stack 400 configured inaccordance with an embodiment of the present disclosure. FIG. 4B is anenlarged view of a portion of FIG. 4A. Referring to FIGS. 4A and 4Btogether, the transducer stack 400 includes a first layer 420 betweenthe transducer 201 and the second layer 224. As shown in FIG. 4A, thefirst layer 420 can have top surface underlying a bottom surface of thesecond layer 224 (e.g., an acoustic lens or window). The first layer canalso have a bottom surface overlying a top surface (e.g., the membrane108 and/or the top electrode 104 of FIG. 1) of the transducer 201. Thefirst layer 420 can be a matching layer configured for use withultrasound having a matrix material 427 doped or filled with a first setof particles 428 (hereinafter “first particles”) and a second set ofparticles 429 (hereinafter “second particles”). For example, in someembodiments, the matrix material 427 can be made of a compliant material(e.g., a PDMS-type silicone, an elastomer, a fluid, and/or any suitablelow-stiffness material having a relatively low Young's Modulus (e.g.,less than 100 MPa)), and the transducer 201 can be configured as a CMUTtransducer as described with reference to FIG. 1. In other embodiments,however, the matrix material 427 may be made of an epoxy or othersuitable material and the transducer 201 may be configured as a PZTtransducer configured for use with ultrasound.

As explained in more detail in the U.S. Pat. No. 8,343,289, incorporatedherein by reference in its entirety, the first particles 428 and/or thesecond particles 429 can be separately selected based on desiredoperating parameters (acoustic impedance, acoustic attenuation,electrical conductivity, density etc.) of the first layer 420. In someembodiments, for example, the first particles 428 may comprisemicron-sized particles (e.g., greater than or equal to one micron) of asuitable first metal (e.g., tungsten, gold, platinum, alloys thereof,and/or a mixture thereof) and the second particles 429 may comprisenano-sized particles (e.g., less than one micron) of the first metal ora suitable second metal (e.g, tungsten, gold, platinum, etc.). In otherembodiments, for example, the first particles 428 and the secondparticles 429 may be made from the same material.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” As used herein, the terms “connected,”“coupled,” or any variant thereof means any connection or coupling,either direct or indirect, between two or more elements; the coupling orconnection between the elements can be physical, logical, or acombination thereof. Additionally, the words “herein,” “above,” “below,”and words of similar import, when used in this application, refer tothis application as a whole and not to any particular portions of thisapplication. Where the context permits, words in the above DetailedDescription using the singular or plural number may also include theplural or singular number respectively. The word “or,” in reference to alist of two or more items, covers all of the following interpretationsof the word: any of the items in the list, all of the items in the list,and any combination of the items in the list.

The above Detailed Description of examples of the disclosed technologyis not intended to be exhaustive or to limit the disclosed technology tothe precise form disclosed above. While specific examples for thedisclosed technology are described above for illustrative purposes,various equivalent modifications are possible within the scope of thedisclosed technology, as those skilled in the relevant art willrecognize. For example, while processes or blocks are presented in agiven order, alternative implementations may perform routines havingsteps, or employ systems having blocks, in a different order, and someprocesses or blocks may be deleted, moved, added, subdivided, combined,and/or modified to provide alternative or subcombinations. Each of theseprocesses or blocks may be implemented in a variety of different ways.Also, while processes or blocks are at times shown as being performed inseries, these processes or blocks may instead be performed orimplemented in parallel, or may be performed at different times. Furtherany specific numbers noted herein are only examples: alternativeimplementations may employ differing values or ranges.

The teachings of the disclosed technology provided herein can be appliedto other systems, not necessarily the system described above. Theelements and acts of the various examples described above can becombined to provide further implementations of the disclosed technology.Some alternative implementations of the disclosed technology may includenot only additional elements to those implementations noted above, butalso may include fewer elements.

These and other changes can be made to the disclosed technology in lightof the above Detailed Description. While the above description describescertain examples of the disclosed technology, and describes the bestmode contemplated, no matter how detailed the above appears in text, thedisclosed technology can be practiced in many ways. Details of thesystem may vary considerably in its specific implementation, while stillbeing encompassed by the disclosed technology disclosed herein. As notedabove, particular terminology used when describing certain features oraspects of the disclosed technology should not be taken to imply thatthe terminology is being redefined herein to be restricted to anyspecific characteristics, features, or aspects of the disclosedtechnology with which that terminology is associated. In general, theterms used in the following claims should not be construed to limit thedisclosed technology to the specific examples disclosed in thespecification, unless the above Detailed Description section explicitlydefines such terms.

1-24. (canceled)
 25. An ultrasonic transducer stack comprising: a firstmatching layer having a first top surface and a first bottom surface,wherein the first matching layer comprises a compliant material, andwherein the first matching layer has a first thickness and a firstacoustic impedance; a lens layer having a second bottom surface that istextured overlying the first top surface, a second top surface, and asecond thickness, wherein the lens layer has a second thickness and asecond acoustic impedance; and a transducer layer having a third topsurface underlying the first bottom surface and the second bottomsurface, wherein the transducer layer includes a micromachinedultrasound transducer configured to generate ultrasound at a centerfrequency, and wherein the third top surface comprises an upper membraneof the transducer.
 26. The transducer stack of claim 25 wherein thecompliant material comprises a PDMS-type silicone.
 27. The transducerstack of claim 25 wherein the compliant material has a Young's Modulusof less than 100 MPa.
 28. The transducer stack of claim 25 wherein thelens layer comprises polymethylpentene.
 29. The transducer stack ofclaim 25 wherein the lens layer comprises a thermoset cross-linkedstyrene copolymer.
 30. The transducer stack of claim 25 wherein thefirst thickness is less than the second thickness.
 31. The transducerstack of claim 25 wherein the first thickness is less than ¼ wavelengthof the center frequency.
 32. The transducer stack of claim 25 whereinthe center frequency is greater than 15 MHz.
 33. The transducer stack ofclaim 25 wherein the second bottom surface is textured with a pluralityof ridges and a plurality of grooves therebetween.
 34. The transducerstack of claim 25 wherein the compliant layer comprises a matrixmaterial, and wherein the first matching layer comprises a compositelayer made of a first plurality of micron-sized particles and a secondplurality of nano-sized particles combines with the matrix material. 35.The transducer stack of claim 25 wherein the first plurality ofmicron-sized particles are selected based on a desired acousticimpedance of the composite material.
 36. The transducer stack of claim25, further comprising a second matching layer disposed between thefirst matching layer and the lens layer.
 37. An ultrasound transducerstack for external use, the ultrasound transducer stack comprising: afirst matching layer having a first top surface and a first bottomsurface, wherein the first matching layer includes a first compliantmaterial and wherein the first matching layer has a first acousticimpedance; a lens layer having a second bottom surface that is texturedwith a plurality of ridges and a plurality grooves therebetweenoverlying the first top surface and a second top configured to be placedagainst a subject's skin, wherein the lens layer has a second acousticimpedance different from the first acoustic impedance; and a third layerhaving a third top surface underlying the first bottom surface and thesecond bottom surface, wherein the third layer includes a micromachinesultrasound transducer configured to generate ultrasound.
 38. Thetransducer stack of claim 37 wherein the compliant material is aPDMS-type silicone.
 39. The transducer stack of claim 37 wherein thecomplaint material includes water.
 40. The transducer stack of claim 37wherein the third top surface comprises an anti-corrosive material. 41.The transducer stack of claim 37 wherein the lens layer is removablyattached to the first matching layer.
 42. The transducer stack of claim37 wherein the first matching layer includes a first plurality ofmicron-sized and second plurality of nano-sized particles combined withthe compliant material to form a composite material.
 43. The transducerstack of claim 37 wherein the compliant material includes a fluid. 44.The transducer stack of claim 37 wherein the micromachines ultrasoundtransducer is a CMUT.
 45. The transducer stack of claim 37 wherein themicromachined ultrasound transducer is a PMUT.